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DB buffer Cache/Page-Swap

Hi.. I have some notes on DB Buffer Cache Parameter from a while ago and was wondering about one thing..

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DOC>Notes on DB Block Buffers (Buffer Cache Hit Ratios)
DOC>In turn, the setting of this parameter has a large effect on the buffer cache hit ratio, which you generally want to be above 90%. The hit ratio can be dynamically determined based on the following query: If the hit ratio is below 90%, then it would be advisable to increase DB_Block_Buffers until you obtain a hit ratio above 90%. The implications of the DB_Block_Buffers being set too low are that the least recently used data will be flushed from memory. If another query needs this data, back out to disk we go. I think we can see this will cause I/O and CPU resources to be used.If the value is set too high, swapping will began to occur on the OS and the system may come to a halt.

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In the last sentence, where it says, value high or low, I believe it should be low... What are the effects, if we have too much SGA allocation? My parameters are all looking good, but system is Paging and swapping at 40%. Is it good or bad or normal..

Env: Oracle 817 on AIX 433
DBsize: 150GB and SGA about 375MB.
Another 4 instances at 50GB with 500MB SGA.
Paging and swapping: set to 4.5GB (across 4 disks)
Total Filesystems space: 240G, RAID 1
System is OLTP and have batch runs before business hours.